]'he human dopamine transporter (hDAT) plays a critical role in neurotransmission via its ability to take up dopamine into the presynaptic terminal and thereby terminate dopaminergic neurotransmission. Alteration of dopaminergic neurotransmission has been suggested to contribute to a multitude of pathological conditions including attention deficit hyperactivity disorder (ADHD). While multiple genetic studies have observed an association between hDAT and ADHD, a systematic screen of the hDAT coding region for variants such as single nucleotide polymorphisms (SNPs) that alter protein function in children with ADHD has not been conducted. The focus of this proposal is the functional impact of SNPs on hDAT function and the identification of variants, such as SNPs, in subjects with ADHD. Specific Aims: 1) Biochemically and pharmacologically characterize previously identified nonsynonymous single nucleotide polymorphisms in the human dopamine transporter. Radioligand transport and binding, and cell surface biotinylation will be used to assess the ability of the mutant transporters to properly transport substrates, bind inhibitors and traffic to the plasma membrane. 2) Identify and characterize hDAT coding variants in subjects with attention deficit hyperactivity disorder. DNA will be isolated from subjects' buccal cells, exons will be PCR amplified and then screened for coding sequence variation in a high-throughput manner using pyrosequencing technology. This work will help to further understanding of the functional impact of coding variation in hDAT. In addition, these studies may identify an etiological pathway for ADHD, which may help to generate more effective :liagnostic criteria as well as novel therapeutics.